Shigella is a global infection that is notorious for disseminating rapidly in certain settings. One serotype, Shigella dysenteriae type 1 (S. dysenteriae 1), can cause devastating pandemics with high case fatality rates and thus it has been classified as a Category B priority pathogen with high potential to be used as a biological weapon. There is no available vaccine for Shigella. The development of effective Shigella vaccines has been hampered by a considerable lack of information of the specific determinants of protective immunity to Shigella infection. Because of the limitations imposed by the risks associated with performing challenge studies with wild type S. dysenteriae 1 in clinical trials to advance vaccine development, a non-human primate model is urgently needed. We have already established a challenge model with wild-type S. dysenteriae 1 strain 1617 which, to date, exhibited an attack rate of 100% (6 of 6 cynomolgus macaques challenged with 10e11 cfu intragastrically). Furthermore, we have advanced our understanding of the immune responses elicited following challenge. In this application we propose to continue these studies by addressing the following Specific Aims: (1) evaluate the hypothesis that intragastric immunization with novel attenuated S. dysenteriae 1 mutant strains elicits protection from intragastric challenge with wild type S. dysenteriae 1;(2) evaluate the hypotheses that a defined set of immune responses observed in circulation in cynomolgus immunized with attenuated strains of S. dysenteriae 1 and/or challenged with wild type S. dysenteriae 1 correlate with protection and are representative of those present at effector sites (i.e., mucosal tissues) and secondary lymphoid organs. These translational studies are central to further our understanding of the immunological mechanisms that mediate protection to S. dysenteriae 1 and longevity of the responses to vaccination in humans, (3) To evaluate the effects of immunization of monkeys with attenuated S. dysenteriae 1 strains on the colonic microbiota in stools of monkeys and the impact of the existing microbiota on the observed immune responses and protection from challenge. Finally, we will take advantage of an upcoming trial with the attenuated S. dysenteriae 1 strain CVD 1256 to evaluate the hypothesis that the immune responses observed systemically and locally in humans are similar to those that correlate with protection in cynomolgus macaques (Aims 1 and 2). These studies will provide valuable insights that might accelerate the development of attenuated vaccines for S. dysenteriae 1.